This invention relates to an improvement of a so-called guardrail for preventing unexpected magnetic bubbles from entering into the active region of an ion-implanted magnetic bubble memory device.
In the normal magnetic bubble memory device, there is provided a magnetic bubble propagation track 7 called a guardrail as shown in FIG. 1 so that unexpected magnetic bubbles can be prevented from entering from the external into an active area 6 of the memory device, which comprises a magnetic bubble generator 1, a write major line 2, minor loops 3, a read major line 4 and a magnetic bubble detector 5. For preventing entrance of unexpected bubbles, the conventional guardrail 7 has magnetic bubble propagation tracks of normal T-bar patterns 9 arranged closely without gap so that magnetic bubbles can be propagated only to the outside from the inside of the guardrail 7, i.e. the active area of the memory, as shown by the enlarged diagram of a partial region 8 in FIG. 2.
However, in the device in which a magnetic bubble propagation track is formed by ion implantation (referred to as ion-implanted device), such a conventional configuration of magnetic bubble propagation track cannot serve as a guardrail. The reason will be briefly described referring to FIG. 3 which shows a partial region 8' corresponding to the region 8 of FIG. 2. If a contiguous disk propagation track 10 is provided, the magnetic bubbles propagated from the inside of the guardrail 7', i.e. from the active memory area 6, to the outside through an upper propagation path 11 are propagated back again from the outside to the inside through the lower propagation path 12 which is contiguous to the upper path 11. In other words, the magnetic bubbles circle around the propagation track 10. As a result, the following problem is caused.
When the propagation path 10 is filled with unexpected or unnecessary bubbles, it can no longer serve as a guardrail and furthermore the magnetic bubbles released from the propagation track 10 may enter into the active memory area 6.
Yet further, if the distance between a pair of adjacent propagation tracks 10 is not small enough, the magnetic bubbles can penetrate through the gap between the propagation tracks. In order to prevent this leakage, it is necessary to construct a guardrail with very closely disposed propagation tracks. This makes the manufacturing process very troublesome.